Reporting from San Francisco and Los Angeles -- A federal appeals court has declared California's 2008 voter-approved ban on same-sex marriage unconstitutional, concluding that the prohibition served no purpose other than to "lessen the status and human dignity of gays and lesbians."

The 2-1 ruling by the U.S. 9th Circuit Court of Appeals was narrowly written to limit its scope to California's borders and possibly even avoid review by the U.S. Supreme Court, legal experts said. Nonetheless, gay-rights advocates hailed Tuesday's decision as historic, while supporters of Proposition 8 immediately vowed to appeal.

Instead of expanding the constitutional rights of gays and lesbians, the court based its decision on a 1996 U.S. Supreme Court precedent that said a majority may not take away a minority's rights without legitimate reasons.

"Proposition 8 operates with no apparent purpose but to impose on gays and lesbians, through the public law, a majority's private disapproval of them and their relationships," Judge Stephen Reinhardt wrote for the court.

The ruling won't take effect immediately; supporters of Proposition 8 have two weeks to appeal to the circuit court and 90 days to file a petition for Supreme Court review.

Though divided on the constitutional question, the three-judge panel unanimously agreed that ProtectMarriage, the backers of Proposition 8, had the right or legal "standing" to appeal Chief U.S. District Judge Vaughn R. Walker's 2010 ruling against the ballot measure.

The panel also unanimously rejected a challenge by ProtectMarriage that Walker's ruling should be set aside because he failed to disclose that he was in a long term same-sex relationship. Walker, who has since retired, ruled after an unprecedented, two-week trial that examined the meaning of sexual orientation and the history of marriage and gay rights.

"It's no surprise that the 9th Circuit's decision is completely out of step with every other federal appellate and Supreme Court decision in American history on the subject of marriage," said Andy Pugno, a lawyer for ProtectMarriage. "Ever since the beginning of this case, we've known that the battle to preserve traditional marriage will ultimately be won or lost not here, but rather in the U.S. Supreme Court."

But other lawyers and legal scholars said the 9th Circuit might have the final word on Proposition 8 because the ruling was so pointedly limited to California, a state where voters stripped a minority of a right that already existed and where the usual justifications for a same-sex marriage ban, responsible parenting and procreation, are undercut by domestic partner laws.

Proposition 8 passed as a constitutional amendment six months after the California Supreme Court struck down a state law that limited marriage to a man and a woman, and an estimated 18,000 same-sex couples married during that time. The initiative also did not affect parenting rights of gays and lesbians, which are protected under other state laws.

"That legal background does not exist in most states," said University of Minnesota Law School professor Dale Carpenter, who has followed the case.

Loyola law professor Douglas NeJaime agreed, noting that Tuesday's decision allows the U.S. Supreme Court to postpone a pronouncement on same-sex marriage until a more sweeping case comes along.

"The 9th Circuit decided the case in a way that would allow the Supreme Court to affirm without having to significantly expand on its existing jurisprudence and without having to rule on marriage for same-sex couples on a national scale," NeJaime said.

ProtectMarriage could ask a larger panel of the 9th Circuit to review Tuesday's ruling, which could keep the case in the circuit for another year. If the group went directly to the Supreme Court and won review, the high court could rule on the case next year.

ProtectMarriage has long said it wanted the high court to get the case as soon as possible, but its representative said Tuesday that the organization has yet to decide its next step.

In the opinion, Reinhardt drew close parallels between Proposition 8 and a 1992 Colorado initiative that barred the government from passing laws to protect the civil rights of gays and lesbians. The U.S. Supreme Court, in a decision written by Justice Anthony M. Kennedy, struck down Colorado's law in 1996.

Calling Proposition 8 "remarkably similar" to the Colorado initiative, the 9th Circuit said both measures singled out one class of people and removed an existing right without serving any reasonable purpose.

"It is implausible to think that denying two men or two women the right to call themselves married could somehow bolster the stability of families headed by one man and one woman," the court said.